Opisthobranchia (Mollusca, Gastropoda) – more than just slimy slugs. Shell reduction and its implications on defence and foraging

Heike Wägele¹^,2 and Annette Klussmann-Kolb³

¹Museum König Bonn, Adenaueralle 160, 53113 Bonn, Germany

²Institut für Evolutionsbiologie, Rheinische Friedrich-Wilhelms-Universität, An der Immenburg 1, 53121 Bonn, Germany

³ Zoologisches Institut, J.W. Goethe Universität, Siesmayerstrasse 70, 60054 Frankfurt, Germany

author email corresponding author email

Frontiers in Zoology 2005, 2:3doi:10.1186/1742-9994-2-3


Published:	16 February 2005

Abstract

Background

In general shell-less slugs are considered to be slimy animals with a rather dull appearance and a pest to garden plants. But marine slugs usually are beautifully coloured animals belonging to the less-known Opisthobranchia. They are characterized by a large array of interesting biological phenomena, usually related to foraging and/or defence. In this paper our knowledge of shell reduction, correlated with the evolution of different defensive and foraging strategies is reviewed, and new results on histology of different glandular systems are included.

Results

Based on a phylogeny obtained by morphological and histological data, the parallel reduction of the shell within the different groups is outlined. Major food sources are given and glandular structures are described as possible defensive structures in the external epithelia, and as internal glands.

Conclusion

According to phylogenetic analyses, the reduction of the shell correlates with the evolution of defensive strategies. Many different kinds of defence structures, like cleptocnides, mantle dermal formations (MDFs), and acid glands, are only present in shell-less slugs. In several cases, it is not clear whether the defensive devices were a prerequisite for the reduction of the shell, or reduction occurred before. Reduction of the shell and acquisition of different defensive structures had an implication on exploration of new food sources and therefore likely enhanced adaptive radiation of several groups.